The present invention relates to a key telephone system which has a function of announcing an off-hook state.
In general, an off-hook announcing function in a telephone system is well known as one means for a third party C to inform to any of users A and B in mutual voice communication, i.e., to enable an off-hook announcement.
A prior art key telephone system of a type for realizing this sort of off-hook announcing function is schematically shown in FIGS. 1 to 3.
More in detail, FIG. 1 is a block diagram of the prior art key telephone system which comprises a main unit and a plurality of key telephone sets connected to the main unit to realize the off-hook announcing function, FIG. 2 is a diagram for explaining flows of voice signals (operational principle) of the system, and FIG. 3 is a block diagram showing a schematic arrangement of a key telephone having the off-hook announcing function (exclusive off-hook announcement telephone set or telephone set having an off-hook announcement adapter connected thereto).
The main unit 1 in FIG. 1 includes a time-divisional time switch (TSM) 2 for performing a switching operation over a PCM signal on a time divisional basis, digital telephone set interface circuits (DECs) 3 for forming an interface with a digital telephone set, and a PCM highway 4 through which the time divisional time switch 2 is connected to the digital telephone set interface circuit 3. Connected to the main unit 1 is a key telephone set (TEL.A) 5 having the off-hook announcing function through a so-called standard line 6 for realizing the fundamental functions and through an additional line 7 for the off-hook announcing function. Also connected to the main unit 1 are standard key telephone sets (TEL.B and TEL.C) 8 through the standard lines 6.
In FIG. 2, reference numeral 9 denotes a receive PCM highway from which a receive PCM signal is input to the time divisional time switch 2, 10 send PCM highway through which a send PCM signal is output, 11 a time slot arrangement of the receive PCM signal, 12 a time slot arrangement of the send PCM signal. Reference symbols A to C in the receive PCM signal represent received voice signals from telephone sets TEL.A to TEL.C and A to C in the send PCM signal represent sent voice signals to telephone sets TEL.A to TEL.C, respectively.
Shown in FIG. 3 is a detailed arrangement of a key telephone set 13 having the off-hook announcing function. More specifically, the off-hook announcement key telephone set 13 includes a pair of circuit block 17 and a control circuit (microprocessor) 15. Each of the circuit blocks 17 has a digital transmission interface circuit 17 formed as a digital transmission interface with the main unit 1 and a codec 15 connected to the associated digital transmission interface circuit 14. Each of the codecs 15 performs an analog/digital conversion over a voice signal. One of the circuit blocks 17 is connected through the standard line 6 to the main unit 1, while the other corresponding to the off-hook announcing function is through the additional line 7 to the main unit 1 as already explained above. Also included in the key telephone set 13 is an analog signal addition circuit 18 which adds together a usual speech signal received from one codec 15 and an off-hook announcement voice signal received from the other codec 15 of the circuit block 17 corresponding to the off-hook announcing function to form an addition signal and which selectively sends the addition signal through a speech-channel select switch 21 to one of receiver amplifiers 19 for a handset 22 or to one of amplifiers 20 for a monitor loudspeaker 23. Further, reference numeral 24 denotes a talk-back microphone, 25 a display, 26 a ten key panel.
Explanation will next be made as to the operation of the above prior art arrangement.
With the key telephone system of such an interconnection arrangement having the main unit and key telephone sets as shown in FIG. 1, when the off-hook announcement key telephone set 13 shown in FIG. 3 is connected in the arrangement to realize the off-hook announcing function, the system is operated as follows.
That is, assume now that the key telephone set A (TEL.A) having the off-hook announcing function and the standard key telephone set B (TEL.B) are in their mutual talk mode. Under such a condition, an off-hook announcement from another standard key telephone set C (TEL.C) can be carried out by sending an off-hook announcement voice signal from the telephone set C through the additional line 7 to the key telephone set A (TEL.A) having off-hook announcing function. The then flow of the voice signal is as shown in FIG. 2.
More in detail, the telephone set A (TEL.A), which has a time slot (TS.sub.n) for usual speech and a time slot (TS.sub.n+1) for the off-hook announcement, links the off-hook announcement voice signal received from the telephone set C (TEL.C) to the off-hook announcement time slot (TS.sub.n+1) under the like control of the time divisional time switch 2, whereby the key telephone set A (TEL.A) of the off-hook announcing function can listen to the off-hook announcement voice signal from the telephone set C (TEL.C) through the additional line 7 of the off-hook announcing function and the circuit block 17 of the off-hook announcing function.
In this way, even in the prior art key telephone system, when a telephone set dedicated to off-hook announcement attached with a circuit for the off-hook announcing function is added, or when a telephone set connected with an adapter for the off-hook announcement and a line for the off-hook announcing function are added, there can be realized a key telephone system having the intended off-hook announcing function.
However, the aforementioned prior art key telephone system realizing the off-hook announcing function has many problems which include:
a) Impossible realization of the off-hook announcing function with respect to standard key telephone sets or independent telephone sets (all the telephone sets in the system) without their modification.
b) Necessity of an exclusive telephone set for the off-hook announcing function or a standard key telephone set having an additional circuit for the off-hook announcing function.
c) Necessity of additional special wiring works for the off-hook announcement.
d) Increase in the cost of the system caused by the necessity of the exclusive telephone set, special additional circuit and special wiring works.